Electro-optical mode converters are used in optical communications systems to allow rapid modulation of an optical signal in accordance with an electrical signal. In most cases, an input light beam having an input state of polarization (SOP) impinges on and traverses through an electro-optical waveguide subjected to an applied electric field. When the geometry of the waveguide and the applied electric field are such that the input SOP is not aligned with a principal axis of the waveguide, the light beam at the output of the waveguide will generally have an output SOP different from the input one. With proper choice of input SOP, waveguide geometry and applied electrical field, it is possible to have the output SOP orthogonal to the input SOP. The expression “mode converter” usually refers to the case where the input and output SOP's are linear and orthogonal to each other whereas the appellation “modulator” is more general term describing a modulation of the light beam's amplitude and/or phase and/or SOP.
Structurally, an electro-optical mode converter will usually include an electro-optic substrate such as a III-V semiconductor or LiNbO3-based material with a defined optical waveguide and an electrode structure disposed in the vicinity of the optical waveguide. As a voltage is applied to the electrodes, an electric field is generated within the optical waveguide and modifies the optical parameters of the waveguide including the orientation of its principal axes and/or its birefringence. This allows for the modulation of the SOP of a light beam traversing the optical waveguide. The term principal axes of a waveguide, or principal waveguide axes, is used to describe the two directions that are parallel to the dominant electric field components of the two lowest order hybrid modes of the waveguide. The orientation of the principal waveguide axes is dependent on the waveguide's geometry and the principal optical axes of the material forming the waveguide.
U.S. Pat. No. 5,566,257, hereinafter referred to as '257, issued Oct. 15, 1996 to Jaeger et al., which is incorporated herein by reference, discloses an electro-optic modulator having an electrode structure with two spaced apart conductive strips disposed on either side of a single semiconductor optical waveguide. Each conductive strip includes projections, or fins, projecting towards the other conductive strip and disposed so as to affect the electro-optical parameters of the waveguide upon a voltage being applied to the conductive strips. At the end of the projections, adjacent the waveguide, are pads.
The electrode structure of '257 is referred to as a “slow wave” electrode structure because it allows a reduction in phase velocity of the microwave signal provided by a voltage signal source connected to the electrode structure and allows for the electromagnetic signal generated at the electrodes to remain substantially in phase with the SOP modulation. The electrode structure of the modulator of '257 can be referred to as a slow wave electrode, and the modulator can be referred to as an electro-optic modulator using slow wave electrodes.
The teachings of '257 provide an electro-optic modulator requiring less electrical and optical power and capable of running at higher frequency than Mach-Zehnder type slow wave modulators such as described in U.S. Pat. No 5,150,436, issued Sep. 22, 1992 to Jaeger et al., which is incorporated herein by reference.
Electro-optic mode modulators such as the ones disclosed in '257 are prone to problems caused by the presence of free carriers, i.e. conduction electrons or holes, in the semiconductor epilayer. These free carriers are present, even though the epilayer is not intentionally doped. The free carriers accumulate under the electrodes when an electric field resulting from a voltage being applied to the electrode structure appears in the modulator. This accumulation of free carriers leads to a screening of the electric field in the waveguide which leads to a less effective modulator. The screening of the electric field is most pronounced at low frequencies, making the modulator difficult to bias to a desired operating point. Furthermore, in cases where a large biasing field is required to overcome modal birefringence between the TE and TM-like modes, the field screening problem is compounded. As free carriers build up, they diminish the effect of the electric field on the waveguide. This is typically compensated for by adjusting the DC bias voltage to increase the strength of the field to offset the free carrier related reduction in field strength.
It would thus be desirable to have a mechanism that would alleviate the problem caused by the presence of free carriers.
Another problem with electro-optic modulators such as described in '257 is that related to the presence of higher order optical modes in the waveguide. The presence of these higher modes lead to unpredictable behaviour depending on the amount of power coupled into the higher order modes. Higher order modes can result in degraded extinction ratio and transfer function instability.
Furthermore, in the device described in '257, the voltage required for mode conversion depends, amongst other factors, on the distance separating pads disposed on either side of the waveguide. The smaller the pad separation, the lower the mode conversion voltage will be and, consequently, so will the power requirement of the mode converter. However, decreasing the pad separation tends to increase the capacitance of the device thereby lowering the impedance of the electrode structure, which should be maintained at a fixed value, such as 50Ω, for optimum performance. Moreover, the mode conversion voltage may by lowered by designing the geometry of the electrode structure to include a high ratio of pad length to pad pitch. However, as in the case where the distance separating the pads is decreased, a higher ratio of pad length to pad pitch leads to an increase in capacitance and a decrease in impedance.
In addition to being concerned about having an electrode structure that attains an optimum impedance, one must also consider how changes in the electrode structure design affect the microwave index of the electro-optic mode converter. The microwave index, which should be similar to the index of the optical mode of the waveguide, increases with increasing capacitance.
The generation of free carriers in semiconductors through absorption of sub-bandgap light is well known and has been documented. A document where this subject matter is addressed is by Vanmaekelbergh et al. in Phys. Rev. Lett. Volume 80, Number 4, Pp. 821-824 (1998). As mentioned above, the presence of free carriers in electro-optic modulators such as disclosed in '257 leads to poor device performance. The most important effect leading to the generation of free carriers by absorption of subband-gap light in unintentionally doped semiconductors is that of two photon absorption, which occurs when a light beam having an energy of at least half the bandgap energy of the semiconductor material impinges on the material and/or propagates therethrough. The number of free carriers generated by the two photon absorption process depends on the optical intensity becomes more problematic as the intensity of the light beam is increased.
It would thus be desirable to have means for alleviating the problem caused by the presence of intrinsic free carriers as well as for eliminating or attenuating higher order optical modes. It would also be desirable to have a mechanism that would allow for a lower mode conversion voltage without leading to a higher than desired capacitance and hence a higher microwave index than the optical index. Finally, it would be desirable to have a waveguide including a material having a bandgap larger than twice the photon energy of the light beam propagating through the waveguide.